Protective coating and method of producing the same



Patented Jan. 24, 1933 UNITED STATES PATIENT OFFICE HORACE A. STAPLES, OF PLAINFIELD, NEW JERSEY, ASSIGNOR TO NATIONAL ELECTRIC PRODUCTS CORPORATION, OF NEW YORK, N. Y., A CORPORATION OF DELAWARE PROTECTIVE COATING AND METHOD OF PRODUCING THE SAME No Drawing.

This invention relates to improved protecting coatings for non-ferrous materials such as brass or other alloys containing copper. More particularly the invention relates to an improved protective coating for brass alloy tubes, parts, plates and the like and to a method for providing tubes and other nonferrous articles with such coatings.

The invention has for one of its objects the provision of an improved protective coating for resisting corrosion which coating improves the appearance of the product, and which coating is adapted for use upon nonferrous articles such as tubes, plates or other parts formed of brass and like alloys.

A further object of the present invention resides in the provision of a novel method of obtaining the desired'coating.

A further object of the present invention resides in the provision of a method of coating brass alloys and like non-ferrous materials by the use of which uniformity of product may be secured at less expense than before and with less loss.

A further object of the present invention resides in the provision of a coating process which will enable coatings to be applied upon articles which heretofore were not readily capable of receiving coatings on account of their composition or proportions and furthermore to provide a coating process which will enable proper coatings to be applied while at the same time preventing over annealing of the alloy during the formation of the coating.

Further and other objects of the present invention will be hereinafter set forth in the specification and claims.

A careful study'of the causes of failure of brass alloy tubes in heat exchangers, condensers, etc., shows that a large percentage of failures are caused by corrosion of the surfaces subjected to the conveyed fluids. To protect these surfaces with a non-corrosive substance would be ideal and very desirable for lengthening the usable tube life. Attempts have been made to accomplish these results heretofore by the use of various nonmetallic compounds, but these results have not met with much success because of the Application filed September 29, 1930. Serial No. 485,313.

lack of uniformity of the film and frequently the compounds have been offsuch nature as to cause considerable loss in heat transfer.

It has accordingly been appreciated that an ideal protective coating would be a metallic oxide coating. Such a coating in general is less soluble in solutions than the corresponding metal or alloy. It is also apparent that with a brass alloy tube the natural choice of a protective coating should. fall on cupric oxide as copper is the predominating metal in the alloy. Such a coating should be an integral part of the tube and would allow the maximum amount of heat transfer. It may be pointed out that a prolonged exposure of a brass alloy to the elements produces a compact film of oxides on the alloys which is known as weathering. The weathering effect may be greatly accelerated by chemical action, also heat applied to the (1) 4Cu plus 0 plus heat equals 2Cu O (red cuprous oxide) (2) 2Cu O plus plus heat equals 4CuO (black cupric oxide) v I (3) 2Cu plus 0 plus heat equals 2CuO.

From the above equations it will be seen that the presence of insufficient oxygen produces the relatively unstable red cuprous oxide which on further oxidation becomes the black cupric oxide. The more desirable 0xide coating would be the black cupric oxide coating which it has been shown can be produced by annealing brass in the presence of suflicient oxygen. This process is sometimes attempted in commercial practice by leaving on the tube or other part the film of oxide produced by the final annealing operation instead of cleaning off the film with acids. Attempts have also been made heretofore toaccelerate this action by dipping the tubes in various compounds. It has been found,

however, that it is impossible with the ordinary annealing furnaces to secure auniform form 5 the interiors of the tubes or portions thereof were coated with a cuprous oxide coating whlch coatlng did not provide the desired proper protection.

To attain improved degrees of perfection in the applied coatings, special furnaces have been provided whereby contmuous uniform heat could be applied in the presence of an excess of oxygen. Variations in the films occurred, however, even when special furnaces were employed and even when previously known compounds were employed to facilitate the formation of the coating.

The a plication of black oxide coatings to brass tu s or like non-ferrous alloy parts also in practice presents other difliculties. Such material for proper heat treatment requires the subjecting of the material to heats of varying degrees of temperature depending upon the initial hardness of the material.

ing or working operations, relatively lower temperatures are required for complete recrystallization, and with relatively lower degrees ofhardness relatively higher tempera tures are required. Time factors are also involved. If a certain time of heat treatment is exceeded at a giventemperature over annealing will occur and the grain structure of the alloy will become too large. Formation of a black oxide requires a given temperature and the time factor to obtain a proper coating will vary with a variation of a mass of the article to be coated, because the high conductivity of the material will carry heat it will not oxidize as properly or as readily as it should to provide a proper coating.

5 the time permitted for heat treatment to ob'- tain proper grain construction in the alloy.

To rovide a proper and relatively unilack oxide coating under the foregoing conditions which are to a certain extent antagonistic to each other, requires a modification to the previously used process which will now be described. In lieu of first anlow, soap, water and oil, then drawing the With higher degrees of hardness after drawaway from the surface to be coated-so thatarticle and subsequently annealing the tube or other part in the presence of an excess of oxygen, the processis modified by adding to the usual drawing compound a quantity of kerosene. In practice the quantity of kerosene which is used may be varied, but sub-- stantially two gallons of kerosene to fifty gallons of drawing compound provides a typical and suitable mixture. The action of the kerosene in facilitating and improving the formation of a black oxide film is not entirely understood, but its use seems to accelerate the formation of the oxide upon the subsequent heat treatment and accordingly, a uniform film of the desired texture and character is produced. Furthermore, with tubes the coating is uniform both inside and outside the tube and the formation of undesired red cuprous oxide coatings is obviated.

The use of kerosene in the drawing compound further seems in some way to accelerate the removal of the zinc from the alloy and its use brings about the formation of the oxide coating in proper time to obviate over annealing in the subsequent heat treatment. The use of kerosene also rovides for the formation of coatings of esired uniform characteristics upon alloys which have so high a zinc content as to render a formation of a proper black'oxide coating difficult if not'impossible heretofore. The use of kerosene also permits the forming of coat-' ings upon thin wall tubes which heretofore .were over annealed during the formation of oxide coatings thereon.

The established practice in the art is to pickle with diluted sulphuric acid after every annealing operation. Such pickhng step is necessar with non-uniform and 1rregular oxide ormations upon the tube. However, according to the present process with some alloys the practice can be adopted of completely eliminating the customary pickling operations after annealing. In fact some parts may be made with no pickling operations whatsoever. Obviously a number of drawing and oxide forming operatlons may be employed.

In other cases with different alloys, pickling steps after annealing are retained except for the final heat treatment and coatmg formation step.

What I claim is:

1. The method of providing a protective coating for the surface of a copper or copper alloy article which includes the step of adding an oxidizing accelerating agent comprising kerosene to the drawingcompound for accelerating the oxidation of the material,

and the step of subsequently, after drawin effecting oxidation by heating the article 1n the presence of an excess of an oxygen.

2. The method of providing a protective corrosion resisting coating upon copper or copper alloy products such as tubing, wherein oxidizing of the surface of the product is efl'ected by a heat treatment after drawing, which includes the step of accelerating the formation of the oxidized coating and providing a uniform coating by adding kerosene to the drawing compound in which the products are immersed prior to drawing, and the step of thereafter applying such coating by an oxidizing heat treatment of the product.

3. A heat treated copper or copper alloy tube with proper grain structure as secured with proper annealing with an interior and exterior of corrosion-resisting black cupric oxide finish of substantially uniform character inside and outside and substantially free from cuprous oxide.

4. The method of providing corrosion resignature.

' HORACE A. STAPLES.

sisting coating upon copper or copper alloy articles which are drawn and thereafter heat treated which includes the step of improving the character of the coating and obtaining uniformity of the coating by adding a petroleum distillate such as kerosene to the drawing compound, thereafter drawing the article and thereafter heat treating it in the presence of an excess of oxygen.

5. A method of providing drawn tubes and like articles of copper or copper alloy-with protective coatings and for heat treating the tubes. or the like, which comprises coordinating the formation of the coating while maintaining the proper heat treatment of the article, by utilizing kerosene with the drawing compound in the drawing operation, for accelerating the formation of a proper and uniform coating during subsequent heat treatment, and by heat treating the article in an excess of oxygen without over-anneal 6. A heat treated part formed of copper or copper alloy and having a protective black cupric oxide coating thereon over its entire surface and having its mass relations such that the part would over anneal if subjected to heat treatment to provide the requisite coating, said part having the grain structure which is provided with proper annealing and having the black cupric oxide coating of uniform and corrosion-resisting character over the surface of the article, said oxide being derived from the alloy itself and oxygen in the presence of heat.

7. The method of ,providing a protective corrosion resisting coating on copper or copper alloy articles, including providing for uniformity of character and complete formation of a protective coating by working the article with a working compound containing an oxide accelerating material comprising kerosene and thereafter oxidizing the surface of the article to provide a uniform oxidized coating thereon by heat treating the article in an excess of oxygen.

8 A heat treated copper or copper alloy article having over its surface a uniform cor- 

